Twitter co-founder Evan Williams has stepped down from his role as CEO of the company, according to a post he authored Monday on Twitter's official blog. Dick Costolo, currently serving as chief operating officer, has taken over, effective immediately; Williams will focus on product strategy.

Williams, already well known in the tech industry for having sold Blogger parent company Pyra Labs to Google in 2003, had co-founded Twitter in 2006 with Biz Stone and Jack Dorsey. Initially, Dorsey served as CEO; he stepped aside almost exactly two years ago, and Williams took over. Dorsey is now on the executive team at e-commerce start-up Square and remains Twitter chairman.

Costolo has been instrumental in developing Twitter's fledgling business model, currently centered on a system of "promoted" tweets, trends, and accounts. It's this business expertise that makes him a better fit for the executive role, Williams explained in the blog post.

"The challenges of growing an organization so quickly are numerous," he wrote. "Growing big is not success, in itself. Success to us means meeting our potential as a profitable company that can retain its culture and user focus while having a positive impact on the world. This is no small task."

As COO, Costolo has become a more prominent face of the company in recent months. It was Costolo who unveiled and detailed Twitter's first concrete business strategy at its Chirp developer conference, in the form of its "promoted tweets" product. Twitter has also said it's working on a suite of tools for businesses to monitor and manage their companies' Twitter accounts. It's also signed search deals with the likes of Google and Bing.

"I completely respect his decision to focus on product strategy and vision," a statement from Costolo read. "As we've all seen with the new Twitter, when [Williams] focuses on product, something amazing can happen. I'm certainly excited to be taking on this role."

Twitter now has 160 million registered users, and 90 million tweets are posted per day, but the service hasn't yet proven its mettle as a sustainable business. It also continues to be plagued by infrastructure issues, from a nasty security flaw that ran amok last month to frequent outages that seem excruciatingly difficult for the company to cure.

In fact, as Williams' blog post announcing his departure went live, Twitter was in the midst of another prolonged hiccup. The company's status blog acknowledged several minutes later that it was experiencing "major site issues."

About the author

Caroline McCarthy, a CNET News staff writer, is a downtown Manhattanite happily addicted to social-media tools and restaurant blogs. Her pre-CNET resume includes interning at an IT security firm and brewing cappuccinos.
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